A marine propulsion system such as an outboard or inboard marine engine is used as a power source for a watercraft such as a boat. Some of the marine propulsion systems comprise: a drive shaft directly coupled to a crankshaft of an engine and provided with a drive bevel gear at its end; a pair of oppositely rotating driven bevel gears which mesh with and are driven by the drive bevel gear; a propeller shaft supported coaxially and rotateably with respect to the pair of driven bevel gears; and a clutch mechanism comprising a dog clutch for causing the propeller shaft to selectively mesh with either of the pair of driven bevel gears. (See for example Japanese Utility Model Laid-Open Publication (kokai) No. 60-163198.)
In the above marine propulsion system, the propeller shaft has a larger diameter portion defining axially outwardly facing shoulder surfaces at its both axial ends so that the shoulder surfaces face and slideably engage the axially inwardly facing surfaces of the pair of driven bevel gears to thereby determine the axial position of the propeller shaft. When rotating commonly with one of the driven bevel gears, depending on the position of the dog clutch of the clutch mechanism, the propeller shaft rotates in the opposite direction with respect to the other one of the driven bevel gears. This relative rotation of the propeller shaft with respect to either of the bevel gears generates a frictional heat and may cause abrasion of the slideably engaging surfaces. These problems are particularly severe with regards to the axially engaging surfaces of the propeller shaft and the reverse driven bevel gear which rotate in opposite directions to each other during the forward operation of the boat, where the traveling speed of the boat, and hence the rotation speed of the propeller shaft, are usually higher than during the reverse operation. Although these slideably engaging surfaces of the propeller shaft and the driven bevel gears usually consist of a so-called thrust metal, such as a sintered metal or a metal coated with synthetic resin, it is still desirable to further improve the durability of the propeller shaft and bevel gears particularly in high power marine propulsion systems in which the rotation speed of the propeller shaft is also high.